On Monday the commission accused the Bulgarian units of Lukoil, Royal Dutch Shell, OMV, Hellenic Petroleum, Gazprom Neft and Bulgarian Petrol, saying they had collaborated to fix retail prices of diesel and gasoline.

Nenkova said she would not disclose details because of commercial and business confidentiality, adding: “I do not think the companies will be able to prove their innocence.”

The Bulgarian Petrol and Gas Association, which represents all the companies being investigated, has said there are no cartel agreements between any of its members and it will appeal on their behalf.

The six companies have 30 days from Oct. 10 to comment on the commission’s findings. They could face fines of up to 10 percent of their annual turnover if found guilty.

The commission began investigating fuel retailers in February after many Bulgarians complained of high fuel costs despite a plunge in global oil prices. The commission raided the offices of the companies in April.

The initial inquiry included Rompetrol, which is not on the list of fuel retailers that the commission says are part of the cartel.

Nenkova also said that the commission has launched analyses of the banking and the pharmaceutical sectors to check if there are reasons for price-fixing investigations.

Reporting by Angel Krasimirov; Editing by Susan Fenton and
David Goodman